Abstract: A packet carried on an optical network is routed by carrying out a logic operation on an address word carried in a packet header and a predetermined discriminator word. A routing decision is made in accordance with the result of the logic operation. The logic operation may be a bit-wise Boolean AND operation. An additional logic operation may be carried out with an additional discriminator word to determine whether the packet is destined for a remote region of the network. In this case, the packet may be steered directly to the remote region.

Abstract: In a method of recognizing an optically encoded binary word, the word is chosen from a subset of binary words for which WT=0 if W=T, WT=1 if W.noteq.T is true, where T is the target word, W is the word to be compared and is the operation formula (I). The word is recognized by carrying out in the optical domain a bit-wise Boolean AND operation. The operation is carried out on the word to be recognized and the complement of the target word. The AND operation may be carried out by introducing the word to be recognized and the target word into a nonlinear crystal and detecting the second harmonic output from the crystal. The complement of the target word may be generated by a fixed word-forming network. The method may be applied to recognizing the address of a packet on an optical network.

Abstract: A multi-bit packet carried on an optical network includes a marker pulse. A bit-rate clock for use in a bit-level operation on the packet is generated by replicating the marker pulse. The bit-level operation may comprise retiming, regeneration or demultiplexing. The marker pulse may be distinguished by a fixed, bit-asynchronous time relationship to the rest of the packet.

Abstract: An optical device includes a 50-50 cross-coupler having a pair of ports optically coupled by a waveguide which includes a portion of material having a non-linear refractive index with a relaxation time such that the effect on the non-linear portion of a first pulse passing through the portion last long enough to affect the phase of a second pulse relative to the first. It finds application as a logic element, optical amplifier modulator and the like.

Abstract: A method and apparatus for generating a plurality of electromagnetic signals, primarily optical signals, having linewidths centered on frequencies fixed relatively to a reference frequency. The apparatus comprises a reference frequency signal source such as a gas laser (1) which generates a reference signal having a frequency f.sub.p. This reference signal is fed to a leading one (3) of a plurality of optical fibre ring lasers (3, 11, 16) connected together in series by optical fibres (6, 12). The characteristics of the injected signal and the form of the waveguide ring (5) defined by the ring laser (3) is such that stimulated Brillouin scattering occurs in the waveguide ring to generate a scattered signal with a frequency f.sub.p -f.sub.A traveling in an opposite direction to the injected signal. This scattered signal is passed by the optical fibres (6) via an isolation device (7) to the next ring laser (11) in series.

Abstract: A method and apparatus for generating relatively narrow linewidth radiation, for example having a 1 MHz linewidth. The apparatus comprises a source of relatively broad linewidth, coherent radiation such as a semiconductor laser (1) generating optical radiation having a linewidth of 10 MHz. The broad linewidth radiation is injected into a waveguide ring (5), the characteristics of the radiation and the form of the waveguide ring (5) being such that stimulated Brillouin scattering occurs in use to generate the relatively narrow linewidth of the order of kHz, coherent wave travelling in an opposite direction to the broad linewidth radiation. A directional coupler (3) between the source (1) and the waveguide ring (5) separates the narrow linewidth wave from the injected radiation.

Abstract: A mode-locked laser light source, i.e. a light source in which resonant longitudinal modes of the laser cavity are synchronized in phase to produce a pulsed laser output, is provided with an optical phase-lock loop, to lock the optical output pulse train to the mode-locking radio frequency drive signal. The invention provides a highly stable laser source for light pulses of picoseconds duration.

Abstract: Stimulated Brillouin Scattering, which occurs when light of narrow linewidth and above a threshold power is launched into a low loss optical fiber, is suppressed by time-varying the phase angle of the transmitted light waves.